The present invention refers to a construction for a reciprocating hermetic compressor of the type having a motor with permanent magnets affixed to a vertical shaft and, more particularly, to a construction for this vertical shaft of the reciprocating compressor.
Compressors having an electric motor with a rotor carrying permanent magnets are used in refrigeration appliances because they allow the angular speed of the compressor shaft to be varied, either continuously or discretely, within a determined rotation range.
The hermetic refrigeration compressors are constructed according to two possible dispositions of the mechanical elements inside the shell thereof. In one of these constructions, the connecting rod-crankshaft system of the compressor is positioned below the electric motor, close to or immersed in the lubricant oil sump existing at the bottom of the compressor shell. In another of these constructions, the connecting rod-crankshaft system is positioned above the electric motor and therefore spaced, by a considerable height, from the lubricant oil sump provided at the bottom of the compressor shell.
Among other factors, the lubrication efficiency of the mechanical system is affected by the amount of oil reaching the movable parts, resulting from the rotation of the compressor shaft, which actuates an oil pump (or centrifugation tube). The oil pumping resulting from the rotation of the tubular vertical shaft generates an oil transportation ascending curve, in the form of an oil parabola, whose upper end should reach or surpass an oil delivery point in the form of a median radial duct provided in the body of the tubular vertical shaft of the compressor in a median Portion of its longitudinal extension.
Thus, the amount of oil available for lubricating the movable parts depends on the centrifugation speed which, on its turn, is equal to the angular speed of the shaft. If the angular speed of the shaft is lower than a predetermined limit value, the end of the parabola will not reach the delivery point, and therefore there will not be enough pumped oil for lubrication. The amount of pumped oil also depends on the required elevation height, which is the height defined between the oil level in the sump and the outlet of said oil at the delivery point.
In order to guarantee an efficient lubrication of the movable parts during the operation of the compressor in low rotations, some known techniques are used.
One of the techniques to guarantee the volume of oil needed to lubricate the movable parts of the compressor is to use a type of construction which foresees the connecting rod-crankshaft system situated below the electric motor. In this case, there is a considerable reduction in the pumping elevation, which is necessary between the oil level in the sump and the delivery point of the lubricant oil (which, in this case, may be the connecting rod-crankshaft system itself).
Another technique for reducing the height between the oil level and the delivery point of the lubricant oil (elevation height) is to add oil to the sump, as a form of increasing its level and, consequently, reduce the elevation height to be overcome.
The lubrication of the movable parts can also be assured by limiting the minimum rotation in which the shaft is operated, in order to assure that a minimum indispensable amount of oil always reaches the desired height.
Using the type of construction in which the mechanical system is positioned below the compressor shell (motor at the upper part) has the disadvantage that the coils of the electric motor do not have a direct contact with the oil in the sump (immersion) and, therefore, the cooling of said coils is less efficient. The poor cooling of the motor may cause degradation of its insulating and conducting materials. Another disadvantage of this technique is that the mechanical system may cause, by the movement of the shaft, a turbulence of the oil in the sump, which may generate noise during operation.
A deficiency in using the technique of increasing the oil level in the compressor sump is the higher cost of adding more oil. Another disadvantage of increasing the oil level is associated with the possibility of this oil contacting the lower surface of the rotor during the compressor operation, producing whirl and foaming of the lubricant oil. This foaming causes deficiency in the lubrication and increases the power consumption for the compressor operation.
The technique of limiting the minimum rotation in which the shaft operates, in order to guarantee that a minimum indispensable oil amount always reaches a desired height has, as an intrinsic disadvantage, the limitation of the rotation range in which the compressor may operate.
Thus, it is an objective of the present invention to provide a reciprocating hermetic compressor in which, independently from the positioning of its motor assembly inside the shell, occurs an adequate lubrication of the movable parts of the compressor needing lubrication, even in low rotations and with no need for increasing the oil level in the sump defined at the bottom of the compressor shell, or for using additional parts other than those already existing in the pumping system of said compressor.
This and other objectives are attained by a reciprocating hermetic compressor, including a hermetic shell, which defines at its bottom a lubricant oil sump and which lodges a cylinder block supporting a tubular vertical shaft having at least a median radial duct of oil passage and carrying at the bottom thereof an oil pump immersed in the lubricant sump, the oil being pumped upon rotation of the tubular vertical shaft towards the median radial duct, said tubular vertical shaft being provided, axially and externally to its internal periphery, with at least one axial channel of oil conduction, having a lower end immersed in the lubricant oil being pumped and an upper end opened to a respective median radial duct.